This invention relates to vinyl-containing polysilanes of the general formula EQU [RSi][R.sub.2 Si]
where there are present 0 to 60 mole percent of [R.sub.2 Si] units and 40 to 100 mole percent of [RSi] units and vinyl-containing polysilanes of the general formula EQU [RSi][R.sub.2 Si][R"Si]
where there are present 0 to 40 mole percent [R.sub.2 Si] units, 0.1 to 99.9 mole percent [RSi] units, and 0.1 to 99.9 mole percent [R"Si] units, where R is an alkyl radical containing 1 to 8 carbon atoms, where R" is selected from the group consisting of alkyl radicals of at least six carbon atoms, phenyl radicals, and radicals of the formula A.sub.y X.sub.(3-y) Si(CH.sub.2).sub.z - where A is a hydrogen atom or an alkyl radical containing 1 to 4 carbon atoms, y is an integer equal to 0 to 3, X is chlorine or bromine, and z is an integer greater than or equal to 1, and where the remaining bonds on silicon are attached to other silicon atoms and vinyl groups. These vinyl-containing polysilanes are prepared by reacting a polysilane of general formula EQU [RSi][R.sub.2 Si]
where there are present 0 to 60 mole percent [RSi] units and 40 to 100 mole percent [R.sub.2 Si] units or a polysilane of general formula EQU [RSi][R.sub.2 Si][R"Si]
where there are present 0 to 40 mole percent [R.sup.2 Si] units, 0.1 to 99.9 mole percent [RSi] units, and 0.1 to 99.9 mole percent [R"Si] units and where the remaining bonds on silicon are attached to other silicon atoms and bromine or chlorine atoms, with a vinyl-containing Grignard reagent or vinyllithium under carefully controlled reaction and process conditions. The reaction conditions must be carefully controlled to ensure that the vinyl groups, which are incorporated into the polysilane via a derivatization reaction, survive the reaction and processing steps intact.
This invention also relates to a method of preparing such polysilanes under carefully controlled conditions to ensure the vinyl groups survive the reaction process. This invention further relates to the silicon carbide ceramics prepared from such vinyl-containing polysilanes. The vinyl-containing polysilanes of this invention may be rendered infusible thermally under an inert atmosphere or by exposure to UV irradiation in an inert atmosphere prior to pyrolysis to form ceramic material. Such cure mechanisms can result in ceramic materials containing only limited amounts of oxygen. Thermally curing the vinyl-containing polysilane is especially preferred because the actual curing can take place during the pyrolysis step to form a ceramic material. The vinyl-containing polysilanes of this invention can also be cured in oxygen-containing atmospheres but the resulting ceramic material obtained from such air cured polymers will contain increased amounts of oxygen.
Haluska in U.S. Pat. Nos. 4,546,163 (issued October 8, 1985) and 4,595,472 (issued June 17, 1986) claimed to produce vinyl-containing polysilanes by a redistribution mechanism by reacting various disilanes and vinyl-containing silanes in the presence of a redistribution catalyst. More careful work has now determined that the vinyl group itself is not incorporated into the resulting polysilane under the reaction conditions employed. The polysilanes of Haluska from the redistribution reaction of disilanes and vinyl silanes do not contain vinyl groups. This is illustrated in the Comparative Example 1 infra.
Haluska, in the just mentioned patents, also claimed that the vinyl content of his "vinyl-containing" polysilanes could be increased by reacting the "vinyl-containing" polysilane with a vinyl Grignard reagent or vinyllithium. However, it has now been determined that under the conditions Haluska employed during isolation of the end product (i.e. temperatures between 200.degree. and 250.degree. C. for time periods longer than a few minutes) that the vinyl groups will not survive. This is illustrated in Comparative Example 2 infra.
Based on these observations, it is clear that the alleged "vinyl-containing" polysilanes of U.S. Pat. Nos. 4,546,163 and 4,595,472 do not contain vinyl groups. In the examples of both patents, the vinyl content was merely calculated based on the initial reactants and the analyzed byproducts using the assumption that any unaccounted vinyl groups must have been incorporated into the polymer. The vinyl content was not determined experimentally. As indicated in the Comparative Example 1 infra, NMR analysis confirms the absence of vinyl groups in polysilanes prepared by the Haluska method.
This present invention differs from that of Haluska in that the reaction and process conditions under which a chlorine or bromine endblocked polysilane and a vinyl Grignard reagent or vinyllithium are reacted are carefully controlled to ensure the survival of the vinyl groups in the resulting polysilane. The present invention results in vinyl-containing polysilanes which are useful in preparing ceramic materials. The polysilanes of Haluska lack the desired vinyl groups. The presence of vinyl groups in the polysilanes of the present invention are confirmed by NMR analysis.
Baney et al. in U.S. Pat. No. 4,310,651 (issued January 12, 1982) disclosed a polysilane of general formula EQU [CH.sub.3 Si][(CH.sub.3).sub.2 Si]
where there was present 0 to 60 mole percent [(CH.sub.3).sub.2 Si] units and 40 to 100 mole percent [CH.sub.3 Si] units and where the remaining bonds on silicon were attached to other silicon atoms and chlorine atoms or bromine atoms. The polysilane was converted to a beta-silicon carbide containing ceramic at elevated temperatures (about 1400.degree. C.). The polysilanes of U.S. Pat. No. 4,310,651 generally are difficult to handle due to their high reactivity in air.
Baney et al. in U.S. Pat. No. 4,298,559 (issued November 3, 1981) prepared polysilanes of general formula EQU [CH.sub.3 Si][(CH.sub.3).sub.2 Si]
where there was present 0 to 60 mole percent [(CH.sub.3).sub.2 Si] units and 40 to 100 mole percent [CH.sub.3 Si] units and where the remaining bonds on silicon were attached to other silicon atoms and additional alkyl radicals of 1 to 4 carbon atoms or phenyl radicals. Upon heating these polysilanes were converted into silicon carbide containing ceramics in high yields.
Baney et al. in U.S. Reissue Pat. No. Re. 31,447 (reissued November 22, 1983) disclosed polysilanes of the general formula EQU [CH.sub.3 Si][(CH.sub.3).sub.2 Si]
where there was present 0 to 60 mole percent [(CH.sub.3).sub.2 Si] units and 40 to 100 mole percent [CH.sub.3 Si] units and where the remaining bonds on silicon were attached to other silicon atoms and alkoxy radicals containing 1 to 4 carbon atoms or phenoxy radicals. Silicon carbide ceramics were obtained by firing these polysilanes to elevated temperatures.
Baney et al. in U.S. Pat. No. 4,314,956 (issued Feb. 9, 1982) disclosed polysilanes of the general formula EQU [CH.sub.3 Si][(CH.sub.3).sub.2 Si]
where there was present 0 to 60 mole percent [(CH.sub.3).sub.2 Si] units and 40 to 100 mole percent [CH.sub.3 Si] units and where the remaining bonds on silicon were attached to silicon and amine radicals of the general formula --NHR.sup.iv where R.sup.iv is a hydrogen atom, an alkyl radical of 1 to 4 carbon atoms or a phenyl radical. A silicon carbide ceramic was obtained by firing this polysilane to an elevated temperature under an inert atmosphere or under an ammonia atmosphere.
The just discussed U.S. Pat. Nos. 4,310,651, 4,298,599, Re 31,447, and 4,314,956 are hereby incorporated by reference. These polysilanes are further discussed in Baney et al. Organometallics, 2, 859 (1983).
West in U.S. Pat. No. 4,260,780 (issued Apr. 7, 1981) prepared a polysilane of general formula EQU [(CH.sub.3).sub.2 Si][CH.sub.3 (C.sub.6 H.sub.5)Si]
by the sodium metal reduction of dimethyldichlorosilane and methylphenyldichlorosilane. The resulting polysilanes had very high softening points (&gt;280.degree. C.).
West et al. in Polym. Prepr., 25, 4 (1984) disclosed the preparation of a polysilane of general formula EQU [CH.sub.3 (CH.sub.2 .dbd.CHCH.sub.2)Si][CH.sub.3 (C.sub.6 H.sub.5)Si]
by the sodium metal reduction of allylmethyldichlorosilane and methylphenyldichlorosilane. These polysilanes were rapidly gelled with ultraviolet irradiation.
Seyferth et al. in U.S. Pat. No. 4,639,501 (issued January 27, 1987) prepared preceramic polymers by reacting a methylpolysilane of the general formula [(RSiH).sub.x (RSi).sub.y ].sub.n with an organosilicon compound having at least two vinyl groups of the general formula [R.sub.2 (CH.sub.2 .dbd.CH)Si].sub.2 Y, where, for example, Y is O, S, NH, NR, or is absent, using either UV irradiation, thermal energy, or catalysts.
It has now been determined that polysilanes of the general formula EQU [RSi][R.sub.2 Si]
which contain vinyl groups and polysilanes of the general formula EQU [RSi][R.sub.2 Si][R"Si]
which contain vinyl groups may be prepared in good yield. The presence of vinyl groups in the polysilanes is confirmed experimentally. These polysilanes may be pyrolyzed at elevated temperatures in an inert atmosphere to produce silicon carbide-containing ceramics. The polysilanes may be cured, and thus rendered infusible, prior to pyrolysis either thermally or by exposure to ultraviolet irradiation. The thermal cure step may be incorporated into the pyrolysis reaction thereby eliminating a separate cure step.